Composition and method for coating stainless metals



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7 Patented July 23, 1957 COMPOSITION AND ll [ETHOD FOR COATING STAINLESS METALS Edwin W. Goodspeed, Royal Oak, and Gaillard W. Dell,

Detroit, Mich., assignors to Parker Rust Proof Company, Detroit, Mich., a corporation of Michigan No Drawing. Application May 28, 1954, a Serial No. 433,282

12 Claims. (Cl. 148-614) The present invention relates tothe treating of stainless metals to produce adherent protective coatings thereon to facilitate drawing, forming or other working operations. More particularly, the invention relates to the application of oxalate-type coatings to stainless metals selected from the class consisting of iron, chromium, nickel and alloys of at least any two of these metals.

It has been found that adherent phosphate coatings facilitate the cold working of carbon steels and protect the metal, and that adherent oxalate coatings do the same for the so-called stainless steels. Satisfactory oxalate coatings on stainless steel have been produced but the known methods have not been entirely satisfactory because of' noxious odors, bath instability, uneconomical consumption of oxalic acid and erratic coating action on certain types of stainless steels. Among the reasons for this, it is believed, is that accelerators, activators and oxidizing agents are required in order to increase the rate of attack of the oxalic acid on the relatively passive stainless steel. Unlike phosphoric acid, oxalic acid is sensitive to certain of these additives, especially to oxidizing agents. The commonly used oxidizing agents such as sodiumchlorate, hydrogen peroxide and others react so readily with oxalic acid as to require slow continuous or periodic addition of the oxidizing agent to maintain satisfactory bath life. Baths containing the most commonly-used activators, the oxygencontaining sulfur compounds such as thiosulfates, etc., are objectionable because they give off noxious odors and gases during use.

It is an object, therefore, to provide an eflicient method for producing uniform, adherent oxalate coatings on stainless metals. It also is an object to provide a stable bath for such treatment, which does not give off noxious odors.

Other objects will become apparent from the description to follow.

It has been discovered that uniform, hard and tightly adherent coatings are produced when clean stainless metal selected from the class consisting of iron, chromium, nickel and alloys of at least any two of these metals are immersed in or contacted with an aqueous bath consisting essentially of water, oxalic acid, an effective amount of at least one activating ion selected from the class consisting of halide ion and thiocyanate ion and, as an oxidizing agent, an effective amount of one or more organic compounds which contain a nitro group. Such a bath is stable at all operating temperatures ranging from room temperature to about 190 F. and does not give oif noxious odors. The processing time is well within feasible commercial practice, for example, contact times of from 1-10 minutes produce uniform, heavy and adherent coatings.

It is believed that the simple bath of this invention, consisting essentially of the three-named ingredients, works efiiciently because the organic nitro compound is a stable oxidizing agent which does not react to any substantial extent with the oxalic acid yet markedly increases the rate of chemical attack on the metal.

By the term stainless metals is meant the stainless or corrosion-resistant metals selected from the class consisting of iron, chromium, nickel and alloys of at least any two of these metals. Thus, while iron in itself is not corrosion-resistant, when combined with either or both chromium and nickel, the resulting alloys are notably so. Chromium and nickel in the essentially pure state are quite corrosion-resistant, when combined they are more so, and when both are combined in even minor proportions with major proportions of iron they form the well-known corrosion-resistant stainles steels. Of course, many of these corrosion-resistant metals and alloys contain small proportions (usually less than 5 percent each) of alloying elements such as manganese, tantalum, columbium, titanium, molybdenum, silicon, vanadium, phosphorus and others. Several important types of stainless alloys included in this definition are the austenitic, martensitic and ferritic stainless steels. The austenitic stainless steels contain a major proportion of iron, from 16 to 26% chromium and from 6 to 22% nickel, with the total of the alloying elements being at least 23%. The martensitic steels contain 4 to 18% chromium, and nickel, if present, not more than 2 or 3%. The ferritic stainless steels differ slightly in composition from the martensitic, usually containing greater amounts of chromium, no nickel and a relatively greater variety and higher proportions of other alloying elements. Another type of corrosion-resistant or stainless alloy which may be coated by this method is the nickel-chromium alloys high in nickel and low in iron, such as Inconel X. Other alloys containing major proportions of nickel, minor proportions of chromium and little or no iron are usually resistant to corrosion and are used for making clad steel sheets. Particularly good results are obtained with the method of this invention when applied to the stainless steels. Best results are obtained with the austenitic stainless steels.

A fuller description of the composition and corrosionresistance of these and other alloys may be found in the ASTM Metals Handbook, 1948 or later edition. All of these and others are satisfactorily coated by the bath of this invention.-

The term halide ion, as used herein, includes chloride ion, bromide ion, fluoride ion and iodide ion. The halide ion may be introduced in the form of any soluble hydrohalic acid, or soluble salt thereof, the cation of which does not form complexes or insoluble or sparinglysoluble compounds by reaction with the oxalic acid. A preferred activator is a compound selected from the class consisting of the water-soluble alkali-metal and ammonium chlorides, bromides, fluorides, silicofluorides, borofluorides and thiocyanates. The chloride and thiocyanate ions are most preferred as activating ions in this invention.

Any of the organic nitro compounds having the requisite solubility in the solutions of this invention can be utilized as the oxidizing agent in this invention. In some cases, organic nitro compounds may have a greater solubility in oxalic acid than in plain water, and vice versa. For this reason, the organic nitro compound should have a solubility in the oxalic acid solutions of this invention which will enable the use of an amount having the ability to increase the attack of the solution on the metal. Since many organic nitro-hydrocarbon compounds are sparingly soluble in water and the solutions of this invention, it is desirable in most cases to utilize the compounds which have a solubilizing group such as one or more halide groups, hydroxyl groups, aldehyde groups, sulfonic acid groups, sulfonic acid salt groups, carboxylic acid or carboxylic acid salt groups, and others.

Illustrative compounds which may be utilized include nitromethane, nitroethane, 1,1 dinitroethane, 1 nitropropane, nitrochloropropane, 2 nitroethanol, glycerol mononitrate, glycerol 1,3 dinitrate, phenylnitromethane, 2,4 dinitrophenylacetic acid, nitrobenzene, o-nitrobenzaldehyde, nitrotoluene, m dinitrobenzene, m nitrochlorobenzene, 0-, mand p-nitrophenol, 2,4-dinitro- 1- naphthol, l-nitro- Z-naphthol, 8-nitro Z-naphthol, picric acid, picramic acid, m-nitrobenzoic acid, 2,4-dinitrobenzoic acid, p-nitroaniline, m-nitrobenzene sodium sulfonate, o-nitrobenzene potassium sulfonate, pnitrochlorobenzene, nitronaphthalene sodium sulfonate, nitrourea, nitrourethane, nitroguanidine and others. Preferred compounds are selected from the class consisting of the soluble sodium, potassium and ammonium salts of the nitrobenzene sulfonic acids; the nitrophenols; and nitroguanidine. Because of their greater solubility and satisfactory stability the sodium, potassium and ammonuim salts ofthe nitrobenzene sulfonic acids are most preferred. As indicated above, mixtures of one or more of the above and other organic nitro compounds may be used.

The concentration of the various substances in the bath may vary somewhat within limits. For example, the concentration of the oxalic acid is not critical for extremely strong solutions have given good coatings. An effective range of concentration is from 2% to approximately 30%, although it is preferred to maintain the bath between 7 and 25%, with about 20% being most preferred.

The concentration of the halide ion and a thiocyanate ion may vary quite widely. Good adherent coatings of satisfactory weight are obtained with as little as 1% halide ion or thiocyanate ion and equally' good coatings have been obtained from oxalate solutions saturated with these ions. An efiective range of concentration is from about 2% to about 20%, with about 10% being most preferred. In this case also, mixtures of one or more halide ions and mixtures of one or more halide ions with thiocyanate ion may also be used.

The concentration of the organic nitro compound is, however, fairly critical because certain minimum amounts are required for the production of hard, adherent coatings at satisfactory speed. The minimum concentration varies with the activity of 'the particular organic nitro compound and, of course, to some extent-with the concentration of oxalic acid and halide ion present. For example, in a bath containing 20% oxalic acid and sodium chloride, m nitrobenzene sodium sulfonate (Sitol) in a concentration of 0.0007% gives only a slight trace of coating, 0.04% a heavy trace, 0.07% a fair coating but not entirely uniform' and 0.1% a complete, uniform and adherent coating. Similarly with picric acid and nitroguanidine, the minimum concentration appears to be about 0.03%. Withall of these and other nitrocompounds a practical minimum is: about 0.1% ofa bath soluble nitro-compound. There is no upper limit except the limiting solubility of the compoundin. the solutions of this invention, since satisfactory coatings have been obtained from solutions saturated with the nitro compound.

The bath of the invention is made simply by dissolving the oxalic acid in water, the halide salt added and finally the organic-nitro compound, followed by' stirring until all ingredients are dissolved. Since both oxalic acid and many ofv the nitro compounds are more soluble in'warm or hot Water than in the cold, the solution of these compounds is sometimes. facilitated by warming the bath, for example to l-195 F., and-then cooling, if necessary, before use. The order of adding the ingredientsis'not critical since the latter do not. reactto-any appreciable extent.

The method of the. invention. is carried outby first cleaning the stainless steel by any conventional method such as Wiping, spraying or immersing in a solvent such as mineral spirits, alkali cleaning bath, or an acid pickling bath, followed by a rinsing with clear water and then contacting or immersing the metal in the oxalic acid bath for the required time. The period of immersion will vary, of course, depending on the efiiciency of the cleaning treatment, on'the strength of the bath, and on the temperature of the bath. With the lower concentrations specified above as much as 10 minutes will be required for a satisfactory coating. With the preferred concentrations 1 to 7 minutes will usually sutfice.

The temperature of the bath may vary from room temperature F.) to about 190 F. At temperatures below about F., the time required for the production of a satisfactory coating will be beyond best commercial practice. Temperatures beyond 175 F. do not result in appreciable increases in coating speed and moreover require excessive heat input. It is preferred, therefore, to operate the process at coating temperatures between 115 F. and 175 F. Temperatures of to 170 F. are most preferred.

Example 1 Inone series of examples, a stock solution was utilized containing the following ingredients:

Material Percent W./Vo1.

Oxalie Acidt 20 Sodium Chloride 15 Organic Nitro Compouu Variable.

To portions'ofthe stock solution, various known oxidizing agents were added in varying amounts and the weight of dust coat and adherent coat on the steel determined. Several organic nitro compounds also were utilized in a similar fashion. In every case, a stainless steel known as type 302, containing 17.5 to 20% chromium, and 8 to 10% nickel, was immersed for two minutes while maintaining the bathat F. The data are summarized below.

In the above comparative data, it may be noted that the strong inorganic oxidizing agents, i. e. nitrate, nitrite and chlorate did. not produce a sufficiently heavy or adherent coat for metal deformation operations. The organic nitro compounds, picric acid and sodium m-nitrobenzene sulfonate, however, deposited heavy coatings which were. uniform and exceptionally adherent to the stainless steel. With the latter, it should also be noted, the total coating was almost twice as heavy as with the strong inorganic oxidizing agents.

Moreover, with chlorate, nitrite and hydrogen peroxide, excessive reaction with'the oxalic acid was noted. Similar results are obtained with about 1% or less of potassium broniate. It was necessary to add these strong oxidizing agents slowly to the bath to minimize decomposition and maintain bath life.

As further examples, the same stock solution, as above, was utilized. As oxidizing agents, sodium m-nitrobenzene sulfonate,, nitroguanidine and picric acid were employed. The concentration of sodium m-nitrobenzene sulfonate was varied from 0.04%, 0.07%, 0.1%, 2% and saturation (about 30%). The concentration of nitroguanidine employed was varied by increments of 0.003% up to 0.03% and again at saturation (less than 4%). The concentration of picric acid employed was varied from 0.003, 0.010, 0.027% and saturation (less than 4%). In every case, type 302 stainless steel was immersed for 5 minutes at 170 F. With sodium m-nitrobenzene sulfonate, hard, adherent coatings of satisfactory weight were obtained with 0.07%, 0.10% and at saturation. With nitroguanidine and picric acid, similarly good coatings were obtained at, respectively, 0.03% and 0.027%, and at saturation. With 2% of each of nitromethane, nitroethane, l-nitropropane, nitrobenzene, o-nitrobenzaldehyde, p-nitrophenol, p-nitroaniline, and p-nitrochlorobenzene, excellent uniform coatings were obtained under commercially practical operating conditions. Thus, while efliceint coating requires certain minimum concentrations of the organic nitro compound, the bath is stable and insensitive to variations above such minimum. From this, it was concluded that at least about 0.1% of any bath soluble organic nitro compound would function efiiciently in the method of this invention.

Example 2 A solution containing 20% oxalic acid, 15% sodium thiocyanate and 2% picric acid was utilized to coat type 302 stainless steel. A panel of this steel was immersed in the solution for five minutes at 170 F. A very good quality, uniform and adherent coating on the metal was obtained.

Unless otherwise specified, the proportions and concentrations used herein are percent weight/ volume. In other words, the density of the solution is taken is unity.

While there have been disclosed certain preferred ways of carrying out the invention, it is desired not to be limited solely thereto, and it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or fair meaning of the subjoined claims.

What is claimed is:

l. A process for treating the surface of stainless metals selected from the class consisting of iron, chromium, nickel and alloys of at least any two of these metals which comprises the step of contacting a surface of the metal with an aqueous bath consisting essentially of water, oxalic acid, at least one activating ion selected from the class consisting of halide ions and thiocyanate ions, and an organic compound which contains the nitro group in an amount sufficient to increase the ability of said bath to form a coating on said surface, for a time suflicient to form an adherent protective coating thereon and at a temperature within the range from approximately room temperature to approximately 190 F.

2. In a process for treating stainless metals as claimed in claim 1, and which is further characterized in that the source of the activating ion is a compound selected from the group consisting of the water-soluble alkali and ammonium chlorides, bromides, iodides, fluorides, silicofluorides, borofluorides and thiocyanates, and the stainless metal is a stainless steel.

3. In a process for treating a stainless metal as claimed in claim 1 and which is further characterized in that the organic nitro compound is selected from the class consisting of the soluble sodium, potassium and ammonium salts of nitrobenzene sulfonic acids; nitroguanidine; and the nitro-phenols and the stainless metal as an austenitic stainless steel.

4. In a process for treating stainless steel to facilitate drawing by means of an adherent protective coating produced thereon, the novel step of contacting the stainless steel in an aqueous bath consisting essentially of water, oxalic acid, alkali metal chloride, and a minimum of about 0.1% of meta-nitro benzene sodium sulfonate, at a coating temperature and for a time sufiicient to form an adherent protective coating thereon.

5. In a process for treating stainless steel to facilitate drawing by means of an adherent coating produced thereon, the novel step of immersing the stainless steel in an aqueous bath consisting essentially of water, oxalic acid, alkali metal chloride and a minimum of about 0.1% of picric acid, at a coating temperature and for a time sufiicient to form an adherent protective coating thereon.

6. In a process for treating stainless steel to facilitate drawing by means of an adherent coating produced thereon, the novel step of immersing the stainless steel in an aqueous bath consisting essentially of water, oxalic acid, alkali metal chloride, and a minimum of about 0.1% of nitroguanidine, at a coating temperature and for a time sufiicient to deposit an adherent protective coating thereon.

7. A solution for coating the surface of stainless metals selected from the class consisting of iron, chromium, nickel and alloys of at least any two of these metals consisting essentially of water, oxalic acid, at least one activating ion selected from the class consisting of halide ions and thiocyanate ions, and an organic compound which contains a nitro group in an amount sufficient to increase the ability of said bath to form a coating on a surface of said stainless metals.

8. A solution for coating the surface of stainless steel consisting essentially of water, oxalic acid, an activating compound selected from the class consisting of watersoluble alkali-metal and ammonium chlorides, bromides, iodides, fluorides, silicofluorides, borofluorides and thiocyanates, and at least about 0.1% of a compound selected from the class consisting of the soluble sodium, potassium and ammonium salts of the nitrobenzene sulfonic acids; nitroguanidine; and the nitrophenols.

9. A solution for coating the surface of stainless steel consisting of water, oxalic acid, sodium chloride and at least 0.07% of m-nitrobenzene sodium sulfonate,

10. A solution for coating the surfaces of stainless steel consisting of water, oxalic acid, sodium chloride and at least 0.03% of nitroguanidine.

11. A solution for coating the surface of stainless steel consisting of water, oxalic acid, sodium chloride and at least 0.03% of picric acid.

12. An aqueous bath for coating the surface of stainless metals selected from the class consisting of iron, chromium, nickel and alloys of at least any two of these metals consisting essentially of water, oxalic acid, at least one activating compound selected from the class consisting of water-soluble alkali metal and ammonium chlorides, bromides, iodides, fluorides, silicofluorides, borofluorides and thiocyanates, and at least one organic compound which contains a nitro group in an amount of at least about 0.1%.

References Cited in the file of this patent UNITED STATES PATENTS 2,431,728 Bergstein Dec. 2, 1947 FOREIGN PATENTS 675,024 Great Britain July 2, 1952 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0.x 2,800,421 July 23, 1957 Edwin Wu Goodspeed at alo It is hereby certified that error appears .in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 50, for "0,0007%" read onoo'rz column 5, line 33, {lot "taken is" read taken as Signed and sealed this 8th day of October 1957 Atte et: KARL H. WINE ROBERT C WATSON Attesting Officer Commissioner of Patents 

1. A PROCESS FOR TREATING THE SURFACE OF STAINLESS METALS SELECTED FROM THE CLASS CONSISTING OF IRON, CHROMIUM, NICKEL AND ALLOYS OF AT LEAST ANY TWO OF THESE METALS WHICH COMPRISES THE STEP OF CONTACTING A SURFACE OF THE METAL WITH AN AQUEOUS BATH CONSISTING ESSENTIALLY OF WATER, OXALIC ACID, AT LEAST ONE ACTIVATING ION SELECTED FROM THE CLASS CONSISTING OF HALIDE IONS AND THIOCYANATE IONS, AND AN ORGANIC COMPOUND WHICH CONTAINS THE NITRO GROUP IN AN AMOUNT SUFFICIENT TO INCREASE THE ABILITY OF SAID BATH TO FORM AN ADHERENT PROTECTIVE COATING THEREON AND AT TO FORM AN ADHERENT PROTECTIVE COATING THEREON AND AT A TEMPERATURE WITHIN THE RANGE FROMAPPROXIMATELY ROOM TEMPERATURE TO APPROXIMATELY 190* F. 